Present day construction of dynamic loudspeakers features a vibrating cone disposed in front of a pole piece with surrounding voice coil. The voice coil is disposed in a magnetic air gap between the pole piece and a front plate. When an audio signal is fed to the voice coil, the voice coil is caused to reciprocate axially within the magnetic air gap about the pole piece.
The non-symmetric magnetic flux interactions in the magnetic air gap during the operation of the loudspeaker have been found to rob the dynamic loudspeaker of output power, as well as to introduce harmonic distortion.
The present invention seeks to shield the voice coil from non-symmetrical magnetic flux interactions produced in the magnetic air gap between the pole piece and the front plate.
Dynamic loudspeakers require a linear magnetic field to reproduce sound with minimum distortion. This requirement becomes most difficult at low frequencies and at high power levels, where there is large amplitude cone and voice coil movement. Improved performance can be achieved by fabricating the pole piece with a stepped configuration, i.e., a pole piece having first and second integral cylindrical sections with an upper cylindrical section having a wider diameter.
In spite of employing stepped pole pieces, the second harmonic distortion has been difficult to eliminate. It is created by the non-symmetric interaction between the magnetic field generated by the voice coil and another magnetic field generated across the air gap through the front plate and the pole piece. This other magnetic field results from a ceramic magnet disposed adjacent the front plate and the pole piece.
Shielding techniques have been employed by others with various success. These prior art shielding methods utilize copper plated pole pieces, or the placement of a copper cylinder through the air gap. Copper plated pole pieces provide only a thin layer of shielding, which is not very effective. Placement of a copper cylinder in the air gap creates a wider air gap, which in turn reduces the power output of the speaker. Another speaker construction has introduced a flux-stabilizing ring located away from the air gap/pole tip and around the pole piece adjacent to the back plate. This ring is claimed to maintain a constraint level of magnetic energy in the voice coil gap.
The present invention has improved the output power and lowered the second harmonic distortion of the speaker by placing two highly conductive, nonferromagnetic members adjacent the magnetic air gap on opposite sides of the wider section of the stepped pole piece. These highly conductive, nonferromagnetic members, in addition to the careful selection of other structural members of the speaker, produces a twelve inch woofer with increased output and low distortion, particularly low distortion in the second harmonic.
The highly conductive, nonferromagnetic members used in the construction of the invention substantially shield the stepped pole piece in the vicinity of the air gap from a non-symmetrical interaction with the magnetic field generated by the voice coil. The sandwich arrangement employed by the invention also effectively reduces voice coil inductance, thereby improving the power output while simultaneously increasing inductance symmetry, thereby reducing non-symmetric distortion. This in turn decreases the audible distortion, especially the second harmonic.